1. Field of the Invention
The present invention relates to an objective lens actuator and, more particularly, to an objective lens actuator which is capable of adjusting an objective lens at a high accuracy and an optical pickup unit employing the same.
2. Description of Related Art
In recent years, optical recording media, hereinafter called optical disks, such as CDs (compact disks), DVDs (digital versatile disks) and Blu-ray disks have been developed to be capable of recording a large capacity of information. The more information the disk is able to record, the higher the recording density is, provided that physical sizes of the different disks are the same. That is, spaces between adjacent tracks in the disks become closer, and the dimensions of the pits become smaller. These changes can be followed in a table as below.
optical disk typespace between adjacent tracksdimension of the pitCD 1.6 μm830 nmDVD0.74 μm400 nmBlu-ray disk0.32 μm149 nm
It is important whether a beam spot emitted from a pickup unit can illuminate on the current track and pit accurately High accuracy is especially crucial to the Blu-ray disk because the space between adjacent tracks is shortened to 0.32 μm. If the beam spot appreciably deviates from the current track, the beam spot may illuminate on the adjacent tracks. As a result, the pickup unit will be unable to gather correct signals based on the beam reflected from the disk.
As is well known, a conventional optical pickup unit generally utilizes an actuator to adjust a position of an objective lens to achieve high accuracy. The actuator moves the objective lens in a radial direction to find a desired track of the disk. Further, the actuator moves the objective lens in an axial direction to adjust the beam spot on desired track and pit.
FIG. 9 (Prior Art) represents a conventional objective lens actuator 100, which is employed in a CD or DVD reproducing/recording apparatus. The objective lens actuator 100 includes a yoke plate 110, a wire holder 120, a plurality of suspension wires 130, and a lens holder 140. On the yoke plate 110, there are two internal yokes 112, two external yokes 114, and two magnets 118. The two internal yokes 112 are symmetrically formed on the yoke plate 110 and are located beside the lens holder 140. The two external yokes 114 are positioned between the two internal yokes 112. The two magnets 118 are mounted on the two external yokes 114, respectively. The wire holder 120 is located at a side of the yoke plate 110. The suspension wires 130 connect the wire holder 120 and the lens holder 140. The suspension wires 130 movably support the lens holder 140. An objective lens 142 is set in the center of the lens holder 140. A focusing coil 144 is horizontally wound around the lens holder 140. Two rectangular tracking coils 146 are attached to opposite sides of the focusing coil 144. The two magnets 118 apply magnetic fluxes to both the focusing coil 144 and the two tracking coils 146. In an operation of the objective lens actuator 100, a current is applied to the focusing coil 144 or the two tracking coils 146 through the suspension wires 130, and, as a result, an electromagnetic force is generated in the objective lens actuator 100 so that the position of the objective lens 142 can be adjusted along with the lens holder 140.
However, the wire-driven objective lens actuator 100, operated by both the focusing coil 144 and the tracking coils 146, is problematic. The system is particularly problematic when the objective lens actuator 100 is used with a Blu-ray disk player for playing a Blu-ray disk having densely packed recording tracks and pits. That is, the movable range of the lens holder 140 is determined by both the focusing coil 144 and the tracking coils 146. Accordingly, it is difficult to adjust the objective lens 142 at a high enough accuracy.
Accordingly, what is needed is an objective lens actuator, which is capable of adjusting an objective lens at a high accuracy. What is also needed is an optical pickup unit employing such an objective lens actuator.